Video duration: 174 seconds
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The roots of a cappella (and the term itself which means "of the chapel") is Gregorian chant and here from Estonia are Hortus Musicus with a fine example of this early music style.
Video duration: 461 seconds
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This is a rendition of the famous 13th century Latin Catholic hymn, "Dies Irae" (or, "Day of Wrath," about the Second Coming of Christ and Judgment Day). This rendition is off the 1994 CD, "Ego sum Ressurectio," and is difficult to find.